It is considered that nickel mattes rich in cobalt will be available from a number of sources, such as, for example, the recovery of metal values from sea nodules, treatment of slags to reclaim metal values from waste products generated in the nickel smelter and various residues arising in the treatment of nickel containing materials.
It is to be appreciated that in the normal operation of the nickel converter wherein iron elimination is desired, that a large proportion of cobalt present in the nickel matte will be oxidized along with the iron. Accordingly, it is to be expected that nickel mattes rich in cobalt will also contain considerable quantities of iron. This factor presents difficulties in terms of potential reagent costs particularly when chlorine leaching is to be employed.
In the case of nickel mattes, which are usually poor in cobalt and iron, chlorine leaching has been demonstrated to have a number of advantages. In particular, the production of chloride solutions provides the means for ready separation of cobalt from nickel based on solvent extraction of the cobaltous chloride complex. Cobalt and nickel electrowinning may readily be conducted from all chloride electrolytes. In applying chlorine leaching to mixed nickel-cobalt mattes containing substantial iron contents, the iron will be converted to ferric chloride during the leaching representing a substantial loss of chlorine. This can only be replaced by hydrolysis of the ferric ions. When large amounts of iron are present, this approach is not practical from the viewpoint of reagent cost.
The leaching of mattes, ores and concentrates with chlorine or oxygen is an established practice is hydrometallurgy. In addition, simultaneous use of chlorine with chloride solutions containing copper is known. Recently, as shown in South Africa Pat. No. 74/3297, replacement of chlorine by oxygen and HCl has been suggested. Methods of removing and recycling copper in solution when leaching nickel matte have been shown in U.S. Pat. No. 3,880,653. The combination of chlorine and ferric chloride solutions has been proposed for leaching nickel matte in U.S. Pat. No. 1,943,337. A method for recycling ferric ions by means of solvent extraction has been suggested in U.S. Pat. No. 4,173,520. The use of both chlorine and oxygen as reagents in a leaching scheme for sulfides has been suggested in U.S. Pat. No. 3,981,962 and has been described in a paper entitled, "Lead-Zinc Extraction from Flotation Concentrates by Chlorine-Oxygen Leaching" at the SME Fall meeting, Salt Lake City, September 1975, preprint No. 75-B-B14 by Scheiner, Smyres and Lindstrom. In accordance with the teachings of these disclosures, oxygen is added simultaneously with or subsequent to chlorine.